The background art includes tether structures, knotless, slipless connections between lines, tether fabrication and deployment, and space systems using tethers.
Tethers
A tether was originally a rope or chain used to fasten an animal so that it grazed only within certain limits. However, there are many specialized tether uses including bungie jumping and weather balloons. Short tethers have been used for decades in space to attach astronauts to their spacecraft.
In 1974 Professor Guiseppe Colombo, holder of the Galileo chair of astronomy at the University of Padua in Italy, proposed using a long tether to support a satellite from an orbiting platform. U.S. Pat. No. 4,097,010, which issued to Professor Colombo and Mario Grossi on Jun. 27, 1978, teaches a satellite connected by means of a long tether to a powered spacecraft. Colombo actively pursued the design of a tethered satellite system.
Several NASA experiments, such as the two Small Expendable Deployer System (SEDS 1 & 2)) and the Plasma Motor Generator (PMG) used tethers in space. SEDS used a nonconducting tether. The PMG used a 500-meter conducting tether. The Tethered Satellite System flights in 1992 and 1996 (TSS-1 & 1R) used a 22,000-meter conducting tether.
On the TSS-1 mission the tether deployed only 260 meters (853 feet) before the deployer failed. On the TSS-1R the tether was deployed 17,000 meters. In the SEDS-2 flight, a 0.8-mm diameter, 20,000-meter long braided single-line tether was deployed to study tether dynamics and lifetime. Orbital debris or a meteoroid severed this tether in less than four days.
Electric potential is generated across a conductive tether in motion across the Earth's magnetic field lines. Electromagnetic forces acting on a conductive tether in orbit can make the tether system behave like an electric motor or generator, to exert useful force to alter the state vector of any mass attached to the tether.
In the TSS-1R flight, the conducting single-line tether was severed after five hours of deployment. This failure was caused by an electric arc produced by the 3,500 volts of electric potential generated by the conductive tether's movement through the Earth's magnetic field.
The Tether Physics and Survivability (TiPS) satellite consists of two end masses connected by a 4,000-meter long non-conducting tether. This satellite was deployed on Jun. 20, 1996 at an altitude of 1,022 kilometers (552 nautical miles). Its tether is an outer layer of spectra 1000 braid over a core of acrylic yarn. The yarn will "puff" its outer braid to two millimeters to "give it a larger cross section to improve its resistance to debris and small micrometeoroids", according to the National Reconnaissance Office (NRO), which is a sponsor of the TiPS mission. As of Jan. 13, 1997 the TiPS tether had survived 207 days.
These experiments, all using single line tethers, illustrate the problem that long-duration and high-value tether missions require a tether capable of surviving single-point failures due to space impactors, electric arcing, or localized material defects.
In 1991, one of the present inventors, Robert Hoyt, invented a lightweight net-like structure that provides many redundant load-bearing paths. A number of primary load bearing lines running the length of the structure are connected periodically by diagonal secondary lines. Where the secondary lines intersect the primary lines, they are firmly connected by knots. The secondary lines are connected only to the primary lines. At either end of the structure, a support ring enforces the cylindrical spacing between the primary lines. The secondary lines are designed with a small amount of slack. These secondary lines are only put under load if a primary line fails. This invention was disclosed in 1992 (Forward, R. L., "Failsafe Multistrand Tether Structures for Space Propulsion", AIAA paper 92-3214, 28.sup.th Joint Propulsion Conference, Nashville, Tenn., 1992 (hereinafter "1992 AIAA Document"). This structure was named a "Hoytether". The term "Hoytether" is used throughout the remainder of this disclosure for this type of structure.
The 1992 Hoytether design teaches that the normally slack secondary lines have half the cross-section (0.707 the diameter) of the primary lines. There are twice as many secondary lines as primary lines, thus the mass of the secondary lines is equal to the mass of the primary lines. In an undamaged Hoytether, the primary lines carry the entire load, while none of the secondary lines are under load.
While the survival probability of a single-line tether decreases exponentially with time, the Hoytether can maintain a high, i.e. greater than 99 percent, survival probability for periods of months or years (Forward and Hoyt, Paper AIAA 95-2890, 31.sup.st Joint Propulsion Conference, July 1995).
Slipless High Strength Line Connections
The Hoytethers and apparatus that use Hoytethers taught by the present invention require that the primary lines be firmly connected to the secondary lines, without slipping, at their crossover junctions. The original Hoytether design did not teach the use of slipless connections. It used knots to connect the primary lines to the secondary lines. Knots cause stress concentrations, which limit the ultimate strength of the overall tether structure. It is essential that the interconnections between the primary and secondary lines be slipless to ensure proper redistribution of the loads when a segment of a primary line fails. The secondary lines in the original Hoytether design were not connected to each other.
Manufacturing companies that make fishing nets use three different knotless, slipless methods of interconnecting two lines. One is used for braided lines; one for twisted lines and one for crocheted lines. Hoytether uses lines braided of three or more smaller lines. These braids must be interwoven where they cross each other to form knotless, slipless interconnections. Alternatively the Hoytether lines may be knitted together. There is a lack of references in the field of braiding. Most of them are instruction books for hand braiding. Appendix B of the Final Report of NASA Contract NAS8-40545 (hereinafter `NASA Report-1995`, which is incorporated by reference), written by the present inventors, provides a summary of braiding fundamentals at pages B-1 to B-5. The information in NASA Report-1995 is proprietary in accordance with FAR 52.227-20 until Jun. 14, 1999. No prior art known to the inventors teaches a method of slipless knotless interconnection between a larger diameter primary line and two smaller diameter secondary lines.
Tether Fabrication
The Hoytether is essentially a tri-axial net structure, with `primary` lines running along the length of the tether and two sets of `secondary` lines connecting these primaries diagonally. They can be made by hand and connected with knots. Because knotted connections severely limit the strength of a structure, it is desirable to use a knotless fabrication technique to achieve interconnections that have strengths approaching the limits of the constituent material. As these tethers may be many kilometers long, fast and inexpensive mechanical methods are required for their practical fabrication.
The present invention may be made by mechanical braiding, i.e. three-dimensional braiding, such as 3-D rotation braiding using braiding machines such as those developed by the Herzog Company in Germany (August Herzog Maschinenfabrik GmbH & Co., Postfach 2260.26012, Oldenburg, Germany. The specialized loom developed by the Nichimo Company of Japan (Nichimo Company Ltd., 2-6-2 Ohtemachi, Chiyoda-Ku, Tokyo, Japan) is used to produce "Ultracross" knotless fishing nets in which the individual strands are braided as a 4-braid line, and the strands are interbraided where they cross. This produces netting that has slipless interconnections that are very strong, approaching the maximum capability of the fiber. Such a loom could, with some modifications, produce the present invention's structure. Only two such machines exist, one in Japan, the other in Washington State. Unfortunately neither can work with the small line diameters needed to practice the preferred embodiment of the present invention. See generally, Ko, F. K., "Braiding", in Engineered Materials Handbook, Vol. 1, Composites. ASM International, Metals Park, Ohio, 1957. Pp. 519-528.
The most common 3-dimensional braiding machines are 4-step braiders based upon the designs of Maistre (German Patent P230-16986, issued 1973) and Forentine (U.S. Pat. No. 4,312,261, issued 1982). Braiding is accomplished by using pneumatics or solenoids to push the parts of the braiding machine to the proper positions. This is a slow process and making a Hoytether kilometers long with these machines would be very time consuming and expensive. The composites division of Albany International (Albany International Research Company, 777 West Street, Mansfield, Mass.) also produces a 3-D braiding machine. This machine uses modular braiding components that are assembled breadboard fashion on a large wall.
While braiding is often used in producing high-strength cables and composite materials, most textile materials are fabricated using knitting and weaving processes. Two such techniques that have the potential for fabricating multiline tether structures include Raschel knitting and 3-D weaving. 3-D weaving requires the use of a cross fiber that adds to the weight, but not to the strength of the tether. The Fukui Company of Japan manufactures knitted netting by a proprietary technique (Fukui Net Company, Ltd., PO Box 119, Island View Drive, Golden Lake, Ont., Canada K0J 1X0.) Fukui produced a sample Hoytether wherein the primary lines were generated as a 4-strand knitted structure and the secondary lines were created by exchanging yarns from one primary to another. Although the secondary lines were interwoven into the primaries, they were exchanged between primaries in such a way that some of the interconnections were not slip-proof joints. As a result, a primary line cut tended to cause this knitted structure to collapse. It may be possible to use Raschel knitting to produce joints that are slipless and could be used to practice the present invention.
Tether Systems
The prior art teaches the use of tethers in space applications. U.S. Pat. No. 5,163,641, issued on Apr. 9, 1990 to Yasaka, teaches the use of a powered spacecraft connected by a tether to a satellite. This tether is disconnected to change the state vector of the satellite. The state of the art of energy and momentum transfer using space tethers is discussed in Ivan Beckey's article "Tethering a new Technique for Payload Deployment", Aerospace America, March 1997, at pages 36-40. Beckey concludes, "Tethers can perform the same functions as propulsive upper stages of direct payload injection, but at lower weight and cost per pound." U.S. Pat. No. 4,923,151, issued Mar. 1, 1988 to Roberts, Wilkinson and Webster, teaches a tether power generator for earth orbiting satellites. U.S. Pat. No. 4,580,747, issued Mar. 15, 1983 to Pearson, teaches use of a long tether extending downward into the atmosphere from a satellite. The state vector of the satellite is changed by forces acting on a lifting body connected to the end of the tether. U.S. Pat. No. 4,824,051, issued Jan. 12, 1987 to Engelking, teaches passing an electric current through a conductive tether attached to a satellite to provide propulsive force to alter the orbit of the satellite. U.S. Pat. No. 5,082,211, issued Jan. 21, 1992 to Werka, teaches use of a tether to deorbit space debris. U.S. Pat. No. 4,727,373, issued Mar. 31, 1986 to Hoover, teaches an orbiting stereo imaging radar system having two spacecraft in synchronous parallel orbits connected by a tether.
The following references are illustrative of the current state of the art in space tethers:
1. Paul A. Penzo and Paul W. Ammann. Tethers in Space Handbook--Second Edition. NASA Office of Space Flight, NASA Headquarters, Washington, D.C. 20546. See the hundreds of references in the 33 page bibliography at the end of the handbook. PA1 2. Joseph A. Carroll, "SEDS Deployer Design and Flight Performance." paper WSEDS-A-1 at the 4th International Conference on Tethers In Space, Washington D.C., April 1995. PA1 3. Robert L. Forward, Failsafe Multistrand Tethers for Space Propulsion, Forward Unlimited, P.O. Box 2783, Malibu, Calif. 90265, July 1992, Final Report on NASA Contract NAS8-39318 SBIR 91-1 Phase I. PA1 4. Robert L. Forward and Robert P. Hoyt, Failsafe Multistrand Tether SEDS Technology Demonstration, Final Report on NAS8-40545 with NASA/MSFC (Jun. 14, 1995). PA1 5. Robert L. Forward and Robert P. Hoyt, "High Strength-to-Weight Tapered Hoytether for LEO to GEO Payload Transfer" Final Report on contract number NAS8-40690 with NASA/MSFC (Jul. 10, 1996). PA1 6. Hans Moravec. "A Non-Synchronous Orbital Skyhook". J. Astronautical Sci., 25(4):307-322, October-December 1977. PA1 7. Robert L. Forward. "Tether Transport from LEO to the Lunar Surface". In AIAA/SAE/ASME/ASEE 27th Joint Propulsion Conference, Sacramento, Calif., June 1991. AIAA Paper 91-2322. PA1 8. Joseph Carroll. "Preliminary Design of a 1 km/sec Tether Transport Facility", Technical Report, Tether Applications, 2603 Crosshaven Lane, San Diego, Calif. 92139, March 1991. Final Report on NASA Contract NASW-4461.